Polyester polymer made in a melt phase manufacturing process contains acetaldehyde, and such polymers subsequently remelted generate additional amounts of acetaldehyde. Acetaldehyde is undesirable because it imparts a noticeable taste, problematic in carbonated soft drink and water packaging. The formation of acetaldehyde is a two-step reaction. In the first step, thermal degradation of the polyester chain results in the creation of acetaldehyde precursors. In the second step, acetaldehyde precursors react to form acetaldehyde. The presence of acetaldehyde (“AA”) in preforms and bottles can be traced to two sources. The first source of AA is produced in the melt phase process for manufacturing the polymer. This class of AA is called residual or free AA and is the actual measurable amount of AA present on or in polyester polymer pellets that have undergone both AA reaction steps in the melt phase for making the polyester polymer. However, in the melt phase process for manufacturing the polymer, thermally degraded polyester chains (first step) produce AA precursors, e.g. species having vinyl end groups, and not all of these AA precursors progress to the second reaction step to form AA in melt phase manufacturing. These AA precursors as discussed further below may, however, react to form AA at a later time upon remelting the polyester polymer pellets to make molded articles.
With all other parameters being equal, the amount of AA generated in the melt phase manufacture and the number of AA precursors made in the melt phase manufacture increases dramatically as the IV (or molecular weight) of the polymer increases. To prevent the build up of AA and AA precursors to unacceptable levels, the polycondensation of the polymer is continued to a limited extent such that the polymer is made to a low IV in the melt phase, solidified, and then further polymerized in the solid state under low oxygen conditions and temperatures sufficiently low enough to prevent the polymer from melting.
The second source of AA is the additional amount of AA generated when the polyester solids are melted in a melt processing zone (e.g. extruder or injection molding machine) by converters to make bottle preforms. AA precursors present in the solids are converted to AA upon under melting conditions to generate more AA than originally present in the solid polyester particles fed to the melt processing zone (second AA reaction step). In addition, the additional melt history in processing zone can result in more thermal degradation of the polyester chain (more of the first AA reaction step); therefore, additional AA precursors can be formed and react to form AA (more of the second AA step). This phenomena is known as AA generation rate. Thus, it is possible to reduce the amount of residual or free AA present in the pellets to a value of 5 ppm or less, or even 3 ppm or less, and yet produce a preform, made in an injection molding machine with a barrel temperature of 285° C. and a melt residence time of about 108 seconds, containing higher levels of AA at 13 ppm. When the preforms are blown into bottles, the high AA levels can adversely impact the taste of the beverage contained in the said bottles.
There are several causes for the formation of residual AA and AA precursors which produce high AA generation rates. One cause is that if the polycondensation catalyst used in the melt phase is not adequately stabilized and/or deactivated in the solid polyester polymer, it can, during re-melting in a melt processing zone, continue to catalyze the conversion of AA precursors present in the polymer to form AA during melt processing. Adequately stabilizing and/or deactivating the polycondensation catalyst, therefore, reduces the amount of AA generated during melt processing (reduces the AA generation rate), even though AA precursors may be present in the melt. While catalyst stabilization and/or deactivation does reduce the AA generated in subsequent melt processing steps, some AA is nevertheless generated by virtue of the heat applied to melt the polymer causing more thermal degradation and by a lower level of catalytic activity that may remain to convert some of the AA precursor species to AA. Moreover, the ease to which catalyst metals can be deactivated differs from metal to metal. For example, Sb metal based catalysts require stronger acids at higher levels to deactivate.
Another cause for the formation of residual AA and AA precursors is the thermal degradation of the polyester polymers in the melt phase which becomes more prevalent as the IV of the polymer is increased at high temperatures. When solid-state polymerization is not used to increase the molecular weight, a longer melt-phase residence time may be necessary to produce the molecular weight needed to blow bottles from preforms having the required properties. This extended melt-phase exposure increases the extent of thermal degradation; therefore, producing PET exclusively in the melt phase with acceptable free AA and/or acceptable AA generation rate during subsequent molding is much more challenging than the conventional scenario where a portion of the molecular build-up occurs in a solid-phase process. Along with a shorter melt-phase step which generates fewer AA precursors, conventional processes have the added advantage of the solid-stating gas sweeping away most of the free AA.
The problem of controlling the presence of AA and AA precursors produced in the melt-phase manufacture was discussed in EP 1 188 783 A2, equivalent to U.S. Pat. No. 6,559,271 B2. This patent proposes that the amount of AA and AA precursors can be limited by keeping the reaction temperature during the entire polycondensation step below 280° C., by using a highly active titanium catalyst at low dosage to limit the residence time of the polymer in the melt-phase manufacture, and by using an excess of AA scavenger added in the melt phase manufacture. Noting that it was particularly important to use highly active catalysts at low reaction temperatures, the use of Sb catalysts was found to be a compromise between reactivity and selectivity, whereas highly active catalysts such as Ti were found to be a better compromise at low dosages and low reaction temperatures. To control AA generation from AA precursors produced in the melt phase manufacture, this patent teaches deactivating the catalyst with a phosphorus compound late toward or after the end of polycondensation so as to allow the catalyst to promote the molecular weight build-up to a intrinsic viscosity (It.V.) of 0.63 dL/g and higher. Finally, the amount of the AA scavenger or binder added must be in excess so as to bind not only the residual or free AA produced in the melt phase manufacture, but to also bind whatever AA is generated in subsequent melt processing steps.
The problem with the approach of using an acetaldehyde scavenger is that they are expensive regardless of when they are added. The problem of adding acetaldehyde scavengers to the melt phase manufacture is that a portion of the scavenger is consumed by the free acetaldehyde present in the melt phase manufacture, thereby requiring the addition of an excess amount of scavenger to bind subsequently formed acetaldehyde. As the amount of acetaldehyde scavenger added in the melt phase manufacture increases, so do costs and the degree of yellow hue imparted to the polymer by the scavenger, especially if the class of scavengers containing amine groups is used. Moreover, the effectiveness of the scavenger may also be impaired by undergoing two heat histories where the polyester is molten, especially when one of the heat histories is under high vacuum, high temperature, and high viscosity conditions (as in the melt phase polycondensation) where the thermal stability of some types of scavenger can be compromised and there can be losses due to scavenger volatility. With some scavengers, the amount of yellow color imparted by the scavenger may increase as the number of melt heat histories increases. It would be desirable, therefore, to produce solid high IV polyester polymer particles which do not contain acetaldehyde scavengers added in the melt phase yet have both a low AA generation rate and low residual acetaldehyde levels when fed to a subsequent melt processing zone.
U.S. Pat. No. 5,898,058 recommends using any one of a large number of conventional polycondensation catalysts (with combinations of Sb catalysts and one of Co, Zn, Mg, Mn or Ca based catalysts exemplified and/or claimed) in which the catalysts are deactivated late. This patent notes that the traditional antimony polycondensation catalyst will begin to catalyze or encourage the degradation of the polymer, leading to the formation of acetaldehyde and yellowing of the polymer. Once the polycondensation reaction essentially reaches completion, further reaction allows the catalyst to degrade the polymer and form acetaldehyde and a yellow hue. The patent discloses the manufacture of polyester precursors at an It.V. of about 0.64 and 0.62 dL/g, or 0.60 dL/g which was increased to an It.V. of 0.81 dL/g by solid state polymerization. The patent notes that solid state polymerization techniques are useful to increase the It.V. of the polyester to these higher levels.
It is known that the production of high IV. polyester polymers in the melt phase is problematic because at high temperature, degradation reactions lead to the formation of acetaldehyde and acetaldehyde precursor formation, and it becomes more difficult to remove AA from the melt as the melt viscosity increases. Consequently, the molecular weight build-up in the melt has in the past been limited to a reasonably low number (e.g. It.V. of about 0.63 or less), followed by further advancing the molecular weight of the polymer in the solid state.
However, it would be desirable to obtain the desired high IV entirely in the melt phase with the elimination of the solid state polymerization step so as to avoid the significant equipment and conversion costs associated with this step. Moreover, high I.V. solid particles produced in the melt phase should have an acceptable AA generation rate for the application without the presence of a substance which binds AA during melt processing to form articles. Preferably, the solids fed to a subsequent melt processing zone should have an acceptable residual acetaldehyde content for the application without the need for adding an excess of an acetaldehyde scavenger to the melt phase production process.